High Level HD combining is not exactly like a Master FM antenna. Yes, the FM signal and the HD signal are combined into one antenna at the output of the two transmitters. Normally, because of the amount of power that has to be wasted in a reject load, HD power is limited to -20dbc (1% of FM power) The biggest Master FM antenna in the area is on the American Tower in Riverview and combines the FM only signals for 93.3, 100.7 & 103.5 with the low level combined FM+HD signals of 88.5, 94.9 and 101.5. 93.3, 100.7 & 103.5 HD signals are on a separate Master HD antenna just below the Master FM antenna. All stations on that tower are at an HD power of -14dbc (4% of FM power) which is the maximum allowed without special authority from the FCC. 97.1 in Holiday is operating at -10dbc (10% of FM power which is the most allowed by the FCC) low level combined into one antenna on the Holiday tower which is only used by 97.1. 97.9 and 105.5 are combined into a Master antenna at the top of the tower. BTW 97.1 has the very First Liquid Cooled Solid State transmitter built by GatesAir. It is a FLX10k, 10kw transmitter running 8.44kw (7.6kw of FM plus 760 watts of HD). 102.5 and 105.5 also have FLX transmitters. 105.5 a FLX20k and 102.5 the very first FLX40k, 40kw transmitter. Cox was instrumental in the design of the FLX transmitters.

I have this question for hdradioeng, rfrus, or anyone:
If a station has a seperate antenna for HD that is at a significantly different height than their FM antenna,
can they run an equivalent ERP to compensate for the height difference?
For example, a station is 1KW @1,000 feet and can run 40 watts HD
Their HD antenna is on the same tower, but only 500 feet up.
Can they now run 160 watts to compensate for the reduced height?

The answer to ai4i is no. If they use a separate antenna, the antenna must already be licensed as an Auxiliary antenna, must basically be on the same tower as the Main antenna and be between 70-100% of the height of the Main FM antenna. You cannot operate more HD power to make up for the lower antenna. But like I previously noted, the HD antenna must be at least 70% of the height of the Main antenna.